There is a method of detecting the state of an eye based on a face image, and measuring the direction of the line of sight of the person to estimate the degree of vigilance of the person. In order to determine the state of an eye, it is necessary to detect an eye part of a face image accurately.
For example, an art of detecting an eye part in a face image is disclosed in Patent Literature 1. First, an upper edge which changes from light (white) to dark (black) color downward from above and a lower edge which changes from dark (black) to light (white) color downward from above are extracted from a face image. Next, a template is created based on an upper-and-lower edge pair, and candidates of an eye are detected through a correlation operation of this template and a face image. When two candidates, one above the other, with high correlation values are detected, they are assumed as an eyebrow and an eye, respectively, with the lower candidate being a candidate of an eye. When the eye candidate blinks, it is considered as a true eye, and the neighborhood area of the eye is stored as a tracking template.
Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. 2000-137792